Numerous industries have long sought to reduce, or eliminate, distortion of panels that have been joined by welding. Weld induced distortion has plagued many industries, including the automobile, aviation, heavy manufacturing, and shipbuilding industries, among others, since the advent of welding.
Such distortion has become an increasingly common problem as manufacturers increase the use of relatively thin plates of material to construct various articles of manufacture in an effort to reduce weight. The thin plates generally must be reinforced with stiffeners welded to the plates to obtain the strength required for a particular application. This stiffener welding introduces residual stresses in the structure and may lead to several modes of distortion, namely, transverse shrinkage, angular change, rotational distortion, longitudinal shrinkage, longitudinal bending, and buckling distortion. Hereinafter usage of the term distortion shall refer to all types of distortion generally, unless indicated otherwise. Thin plates are particularly susceptible to buckling distortion due to their low bending stiffness compared to their membrane stiffness. Buckling distortion is characterized by a wavy undulating surface of the plate. Such out-of-plane distortion is often many orders of magnitude greater than the thickness of the plate and generally leads to the loss of dimensional control and structural integrity.
During welding, a weld region's high temperature causes compressive stress as a result of thermal expansion in the region and the corresponding restraint on the expansion by the surrounding cooler material. The compressive stress in the weld region may then exceed the yield stress of the plate at the elevated temperature. As such, material in the vicinity of the weld plastifies and compressive plastic strains are produced. As the weld cools the stress patterns change from compressive to tensile in the locations that have plastified during the welding, thereby producing residual tensile stress in the weld region.
Buckling distortion occurs if the residual compressive stress in the plate exceeds the critical buckling stress of the assembly. Therefore, stated another way, buckling distortion is a result of the creation of residual tensile stress in the plate along each stiffener and the residual compressive stress in the plate between each stiffener and along any free edge of the plate.
Industry has tried to overcome buckling distortion in a number of ways, both mechanically and thermally. Some industries, including shipbuilding, have simply learned to accept buckling distortion and apply a post-welding procedure to remedy the distortion. The post-welding procedure is often referred to as “flame straightening” and involves the heating of discrete spots of the plate until they are red hot and then quenching the spots with water to reduce the wavy nature of the stiffened plates. Shipyards generally employ an entirely separate class of skilled tradespeople known as flame straighteners to perform this function. Such flame straightening is a trial and error approach that requires tremendous skill and is extremely time consuming. Flame straightening often requires the repainting of flame damaged areas. In fact, studies have indicated that $3.4 million is spent correcting distortion during the construction of each destroyer built for the United States Navy.
Some have tried to overcome buckling distortion using mechanical methods. An example of this has been called “low stress non-distortion (LSND) welding” as reported by Guan, et al., in their paper “Low stress non-distortion (LSND) welding—a new technique for thin materials;” Welding in the World, 33:3, pp. 160-167 (1994). These methods are often referred to as “back bending” and generally include some form of mechanical tensioning. In the method of Guan, et al., a stretching effect is produced by specific temperature distribution while restraining fixtures (e.g., “two-point:clamping”) are used to prevent transient out-of-plane buckling movement of the workpieces. As one with skill in the an can imagine, mechanical tensioning of large plates is impractical for most applications.
The most promising method for overcoming buckling distortion of thin plates is known as thermal tensioning. Thermal tensioning is characterized by the application of auxiliary heat during the welding process. Thermal tensioning is divided into static thermal tensioning and transient, or dynamic, thermal tensioning. Static thermal tensioning is a technique for controlling welding residual stress and distortion by generating tensile stress at the weld zone prior to, and during welding, by imposing a predetermined steady state temperature gradient. Achieving a predetermined steady state temperature gradient requires the use of a combination of heating elements and cooling elements to create a heat sink and achieve the temperature gradient. Heating elements, often in the form of direct fired heaters or resistive heating blankets, are applied on opposing sides of the stiffener location at a predetermined distance away from the stiffener. Cooling is then provided in the immediate vicinity of the proposed weld location and is generally accomplished with the impingement of cool water to the underside of the plate. An exemplar of this technique is seen in the work of Burak, et al, reported in “Controlling the Longitudinal Plastic Shrinkage of Metal During Welding;” Avt. Svarka, No. 3, pp. 27-29 (1977).
Burak, et al., utilized electrical strip heating elements beneath the plate and lateral to the weld line, with a water cooled copper plate below the weld line, to produce the required gradient. Welding the stiffener to the plate takes place once the desired temperature differential is achieved. While carefully controlled small scale laboratory experimentation have shown that static thermal tensioning does reduce the amount of buckling distortion, it is widely accepted that the use of steady state heating and cooling would not be practical in a manufacturing environment due to the time required to reach steady state and the limitations associated with cooling elements.
Transient thermal tensioning utilizes a transient temperature differential generally produced by two heating bands traveling along with the welding torches that are joining the stiffener to the plate. A large amount of research has been performed on the transient thermal tensioning technique, as applied to the reduction of residual stress. This research has primarily focused on determining the appropriate intensity, size, and location of the heat source to minimize welding residual stress, thereby reducing the amount of distortion. More specifically, and most importantly, the research has been directed to reducing the maximum value of the peak stress, i.e. the maximum tensile stress, observed at the stiffeners. A detailed analysis of thermal tensioning to minimize tensile stress is seen in Michaleris and Sun, “Finite Element Analysis of Thermal Tensioning Techniques Mitigating Weld Buckling Distortion;” Welding Research Supplement, November 1997, pp. 451-s thorough 457-s; and Michaleris, et al., “Minimization of Welding Residual Stress and Distortion in Large Structures;” Welding Research Supplement, November 1999, pp. 361-s through 366-s. This focus on reducing the maximum value of the peak stress has not provided the results necessary to make transient thermal tensioning commercially viable.
In contrast, the method of the present invention makes no attempt to reduce the peak stress, but rather focuses on altering the stress pattern. By inducing areas of tensile stress in desirable locations, the present invention achieves the primary goal, that of greatly reducing the propensity of the plate to buckle.
Accordingly, the art has needed a means for minimizing distortion that occurs concurrently with the welding of the stiffeners. While some of the prior art devices attempted to improve the state of the art, none have recognized the importance of achieving a desirable stress pattern to reducing the propensity to buckle. Additionally, the prior art has not been suitable for widespread application in a manufacturing environment. The prior art has failed to achieve the unique and novel configurations and capabilities of the present invention. With these capabilities taken into consideration, the instant invention addresses many of the shortcomings of the prior art and offers significant benefits heretofore unavailable. Further, none of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.